Ethylene polymerisation



- Chemical Industries Limited, London, England, a corporation of, Great Britain No Drawing; Filed Feb. 28, 1956, Ser. No. 568,171

Claims priority, application Great Britain Jan. 16,1956,

.3:Claims... (Cl.26.0.-9.4.9).

This invention relates to a continuous process for making solid polymers from; ethylene by polymerising it under superatmosphericpressures and at elevated temperatures, particularly polymers that are of higher density and stiffness thanthose that have commonly been made hitherto by such a process.

Itisknown that solid polymers of ethylene can be ob,- tained by subjecting ethylene and-Ya suitable catalyst to temperatures in the rang of about 50-400 C; and to pressures above about 500 atmospheres. Many catalysts have been proposed for. suchreactions e.g. oxygen, persalts, organic peroxides, metal alkyls and azo compounds, and with some ofthese, particularly per-salts and peroxides, it is often preferable to carry out the polymerisation in presence of'an inert liquid .mediumeg. water or benzene. The polymers made by mostof these known processes constitutethe polythenes ofv commerceas generally hitherto understood, and though these polymers can never be unequivocally defined in terms of a single molecular weight or someother property since-they are, like most polymeric materials, mixtures of several individuals possessing difierent molecular weights and difierent physical properties, they doin general have densities less than 0.925 gm./cc. There is however a growing interest in polythenes of higher density than 0.925 since such polymers'possess greater. mechanical stiifness, as'measu'red for exampleby bending :modulus determinations, and are accordingly useful for making. into objects requiring greater rigidity.

Polythenes having: densities greater than. 0.925 grn./ cc. are known and some of them have been made*by -polymerising ethylene in processes requiring high pressures, but these processes, have also required the presence. of liquidreaction mediator successful results;to.be.achieved For example a process is known which, whencarried out in an aqueous-organic or aqueous reaction medium, at pressures of 1200-2000 atmospheres and temperatures of 120-300 C. and in the presence of cyclohexane to .con- 7, trol' the chain growth, yields polymers having densities in the range 0.925-0.950.

Itis the principal object ofjthepresent,inventionto provide. a drypolymeris,ation process, that is to say one that does not depend for results on the presence of a liquid reactiontmedium, whereby ethyleneunder the action of high pressures and'moderately elevated temperatures yields solid, stifi and' tough polythenes having densities greater than 0.925, gm./ cc.

According to our invention we provide a continuous process for making solid, stiff and tough polymers. of ethylene havingdensities greater than 0.925 gm./cc. that comprises subjecting ethylene to a temperature below 150 C. and to a pressure above 500 atmospheres, in the presence of a free radical-producing ethylene polymerisation catalyst and from 0.01% to 5.0% by weight ofjthe ethylene of a chain-transferagent, for example hydrogen, carbon tetrachloride or chloroform.

1 We prefer to operate at pressures above. 1250 atmospheres, for example 1500 2,957,860 Patented Oct. 25, 1960 2 atmospheres; the highest-desirable pressures are about 2000. atmospheres.

The choice of a preferred temperature range is influenced, by; thefacts that the higher thertemperature -the higher is the-,rate, of chain initiation but, for a given catalyst, the, lower is the. density and stiffness of the polymer. The higher'thereactivity of the catalyst the lower is the temperatureat which'one can operatezat, a given, rate and the higherthe density and vstifinessqofvthe polymer Thus choiceof catalyst goes hand inhand greater than havepolymers made hitherto in a dryf .high .p 'essure process.

Very suitable combinations. are temperatures of 551l0 C, with catalystsconsisting of peroxydicarbonate esters for example di-isopropylperoxydicarbonate 0, p (CH:).:-.GH9;( 3. 0.- 3 :0- E (CH1):

and di-iso-amyl peroxydicarbonate. Other suitable combina-tions include alkyl and aryl hyponitrites, for example methyl, ethyl, isobutyl and ben zyl hyponitrites at temperatures of 15 -50" C. Yetanother suitable combination includes int-substituted acyl peroxides, for example di-isobutyryl peroxide and di-pivalyl peroxide at temperatures of 2060 C. Suitable proportions of these catalysts are in the range 5-100 parts per million parts of ethylene; useful'results are often obtained with as, low a proportion as IOparts per million. Many of the peroxy compounds known to be eflicient catalysts in the known processes for polymerising ethylene under high pressure to give the polyethylenes known hitherto are insufliciently reactive in theprocess of our invention. Among such peroxy compoundsare benzoyl peroxide, di-tertiary butyl peroxide, lauroyl peroxide.

Oxygen itself "is not a catalyst for the reactions covered by our invention; on the contrary, in amounts greater than about 5 parts per million it interferes with the action of the. catalyst under the conditions of our process, and consequently the ethylene fed to the reaction vessel preferably contains less than about 5 parts per million by weight.

A suitable proportion for the chain transfer agent, hydrogen, is 05-25% by volume of the ethylene measured at atmospheric pressure. It is known to polymerise mixtures of' ethylene and up to; 10% by weight ofhydrogen at pressures of 1000 atmospheres and less, and at elevated temperatures in the presence of water and/or benzene, and withoxygen as catalyst in order to obtain soft, pastelike waxes; .and with benzoyl peroxide as catalyst, and in the absence ofa liquid medium, to obtain a'hard, brittle wax of relatively low molecular Weight, but none of these products is a solid, tough and stiff polymer of ethylene having a density greaterthan 0.925, and the present-invention is. not concerned with such reactions.

A suitable proportion for the chain-transfer, agents carbon tetrachloride and. chloroform is ODS-1.0% by weight of the ethylene. It is sometimes convenient to use a mixtureof carbon tetrachloride and chloroform to prevent freezing of the former 'when being introduced into the reaction vessel.

Other chain-transfer agents may be used in the process of our invention, for examplehexachloroethane; satu rated halogenated carboxylioacids and their esters, for example. dichloroacetic. acid and its ethyl ester; aldehydes, for example acetaldehydegalkyl esters of inorganic oxyacids of sulphur, phosphorus or silicon, for example diethyl. .sulphite, triethyl orthophosphate and tetrabutyl and mercaptans, for example Example 1 Ethylene containing less than 5 parts per million of oxygen was mixed with 2.5% by volume of hydrogen and the mixture compressed to a pressure of 1250 atmospheres and admitted together with 10 parts per million of di-isopropyl peroxy dicarbonate as catalyst to a pressure vessel fitted with a stirrer, means for controlling the temperature, and suitable inlets and outlets-to allow the continuous introduction of reactants and continuous discharge of products. The temperature of the ingoing ethylene was 25 C. and that inside the steel vessel was maintained at 75 90 C. The contact time within the vessel was about 10 minutes and the ethylene/polythene conversion achieved was about 8.3%. A solid polymer of ethylene was continuously produced having 'the following properties:

Density g lcc-.. 0.947 Bending modulus lbs./sq. inch 70,000 Grade number (B.S. Specification No. 1972) (melt index) 0.9

Example 2 The operating conditions were similar to those in Example 1 except that the pressure was 1400 atmospheres and the temperature 5 60 C. The conversion achieved was about 8.0% and the polymer had the following properties:

Density gm /cc 0.950 Bending modulus lbs./sq./ inch 82,000 Grade number (melt index) 0.3

Example 3 i The operating conditions were similar to those in Example 1 except that the proportion of hydrogen was 1.5% by volume of the ethylene. The conversion achieved was about 9.0% and the polymer had the following properties:

Density grn./cc 0.944

Bending modulus .lbs./sq. inch 65,000

Grade number (melt index) 0.02

Example 4 In a similar experiment to the three foregoing, the pressure was 2000 atmospheres, the temperature 50 -60 C., and the proportion of hydrogen 2.5% by volume of the ethylene. The conversion achieved was about 8.2% and the polymer had the following properties:

Density gm /cc 0960 Bending modulus lbs./sq. inch 93,000 Grade number (melt index) 0.1

Example 5 Density gm./cc 0.941

Bending modulus lbs./sq. inch 60,000

Grade number (melt index) 1.0

Vicat softening point C 117 Example 6 i The experiment described in Example 1 was repeated by weight of pentane.

but with di-isoamyl peroxydicarbonate as catalyst in place of di-isopropyl peroxydicarbonate. The solid polymer of ethylene obtained had the following properties:

Density gm./cc 0.948

Bending modulus lbs./sq. inch 70,000

Grade number (melt index) 0.90

Example 7 The experiment described in Example 5 was repeated but with di-isoamyl peroxydicarbonate as catalyst in place of di-isopropyl peroxydicarbonate. The solid polymer of ethylene obtained had the following properties:

Example 8 Into a high-pressure reaction vessel cooled to below 0C. was introduced 0.5 part by weight of a solution containing 5.2 parts of di-isobutyryl peroxide in 100 parts This was sufficient to amount to about 15 parts per million of ethylene in the reaction. The vessel was closed and the air in it removed by repeatedly purging with ethylene containing less than 5 parts per million of oxygen. Ethylene containing 2.0% by volume of hydrogen was then compressed into the vessel up to 1400 atmospheres, the vessel being at the same time heated up to 50 C. and maintained at that temperature as closely as possible. After the pressure had fallen by about 100 atmospheres over 20 minutes the vessel was opened and 5 parts of a white solid polyethylene extracted, which was found to have the following properties:

Density m /cc 0.949 Yield stress .lbs./sq. inch 3045 Bending modulus lbs./sq. inch 79,000 Grade number (melt index) 0.68

Example 9 In a similar experiment to that described in Example 8 di-pivalyl peroxide (bis-trimethyl acetyl peroxide) was used as catalyst instead of di-isobutyryl peroxide, the reaction pressure was 1000 atmospheres and the temperature 20 C. The solid polymer produced has the following properties:

Density gm /cc 0.942 Yield stress 1bs./ sq. inch 3900 Bending modulus lbs./sq. inch 62,000 Grade number (melt index) 0.80

Example 10 In this experiment conditions were the same as those in Example 9 except that as chain-transfer agent carbon tetrachloride was used in place of hydrogen; the proportion was about 0.10% by weight of the ethylene. The solid polymer produced had the following properties:

Density gm /cc 0.950 Bending modulus lbs./sq. inch 80,000 Grade number (melt index) 0.70

Example 11 solid polymer of ethylene was extracted which had the following properties:

In this experiment the conditions were the same as those in Example 11 except that carbon tetrachloride was used as chain-transfer agent in place of hydrogen, the proportion being about 0.10% by weight of the ethylene. The solid polymer produced had the following properties:

Density gm /cc 0.952

Yield stress "kg/cm?" 241 Bending modulus lbs./sq. inch 82,000

Grade number (melt index) 0.84

What we claim is:

1. Process for the manufacture of solid polymers of ethylene having densities greater than 0.925 gram per cubic centimeter which comprises subjecting gaseous ethylene to a pressure between 1000 and about 2000 atmospheres and a temperature between about and C., in the presence of a peroxy dicarbonate ester polymerization catalyst and hydrogen as a chain transfer agent present in an amount equivalent to between 0.5 and 2.5% by volume of the ethylene measured at atmospheric pressure and in the absence of any significant amount of added liquid medium whereby said process is carried out under essentially liquid-free conditions.

2. Process as claimed in claim 1 in which the peroxydicarbonate ester is diisopropyl peroxydicarbonate.

3. Process as claimed in claim 1 in which the peroxydicarbonate ester is di-isoamyl peroxydicarbonate.

References Cited in the file of this patent UNITED STATES PATENTS 2,475,643 Seebold July 12, 1949 2,586,322 Franta Feb. 19, 1952 FOREIGN PATENTS 474,642 Canada June 19, 1951 

1. PROCESS FOR THE MANUFACTURE OF SOLID POLYMERS OF ETHYLENE HAVING DENSITIES GREATER THAN 0.925 GRAM PER CUBIC CENTIMETER WHICH COMPRISES SUBJECTING GASEOUS ETHYLENE TO A PRESSURE BETWEEN 1000 AND ABOUT 2000 ATMOSPHERES AND A TEMPERATURE BETWEEN ABOUT 55* AND 110*C., IN THE PRESENCE OF A PEROXY DICARBONATE ESTER POLYMERIZATION CATALYST AND HYDROGEN AS A CHAIN TRANSFER AGENT PRESENT IN AN AMOUNT EQUIVALENT TO BETWEEN 0.5 AND 2.5% BY VOLUME OF THE ETHYLENE MEASURED AT ATMOSPHERIC PRESSURE AND IN THE ABSENCE OF ANY SIGNIFICANT AMOUNT OF ADDED LIQUID MEDIUM WHEREBY SAID PROCESS IS CARRIED OUT UNDER ESSENTIALLY LIQUID-FREE CONDITIONS. 